1. Technical Field
The present invention relates to an atomic oscillator, a control method of the atomic oscillator and a quantum interference apparatus.
2. Related Art
As shown in FIG. 19, it is known that an alkali metal atom has a ground level represented by a term symbol 2S1/2 and two excited levels represented by term symbols 2P1/2 and 2P3/2. Further, each level of 2S1/2, 2P1/2 and 2P3/2 has an ultrafine structure split into plural energy levels. Specifically, 2S1/2 has two ground levels of I+1/2 and I−1/2, 2P1/2 has two excited levels of I+1/2 and I−1/2 and 2P3/2 has four excited levels of I+3/2, I+1/2, I−1/2 and I−3/2. Here, I is a nuclear spin quantum number.
An atom, which is in the ground level of I−1/2 of 2S1/2, can transit to the excited level of one of I+1/2, I−1/2 and I−3/2 of 2P3/2 by absorbing a D2 ray but cannot transit to the excited level of I+3/2. An atom, which is in the ground level of I+1/2 of 2S1/2, can transit to the excited level of one of I+3/2, I+1/2 and I−1/2 of 2P3/2 by absorbing a D2 ray but cannot transit to the excited level of I−3/2. These depend on a transition selection rule when assuming an electric dipole transition. Conversely, an atom which is in the excited level of I+1/2 or I−1/2 of 2P3/2 emits the D2 ray and can transit to the ground level (one of the original ground level or the other ground level) of I+1/2 or I−1/2 of 2S1/2. Here, three levels (configured of two ground levels and one excited level) configured of two ground levels of I+1/2 and I−1/2 of 2S1/2 and the excited level of I+1/2 or I−1/2 of 2P3/2 are referred to as Λ-type three-level system because the transition of Λ-type can be performed by absorption and emission of the D2 ray. Meanwhile, an atom which is in the excited level of I−3/2 of 2P3/2 emits the D2 ray and necessarily transits to the ground level (the original ground level) of I−1/2 of 2S1/2 and similarly, an atom which is in the excited level of I+3/2 of 2P3/2 emits the D2 ray and necessarily transits to the ground level (the original ground level) of I+1/2 of 2S1/2. That is, three levels configured of two ground levels of I+1/2 and I−1/2 of 2S1/2 and the excited level of I−3/2 or I+3/2 of 2P3/2 do not form the Λ-type three-level system because the transition of the Λ-type cannot be performed by absorption and emission of the D2 ray.
Meanwhile, when a resonant light beam (referred to as a resonant light beam 1) having a frequency corresponding to an energy difference between a first ground level (the ground level of I−1/2 of 2S1/2) forming the Λ-type three-level system and the excited level (for example, the excited level of I+1/2 of 2P3/2), and a resonant light beam (referred to as a resonant light beam 2) having a frequency corresponding to an energy difference between a second ground level (the ground level of I+1/2 of 2S1/2) and the excited level are simultaneously irradiated, it becomes a superimposed state of two ground levels, that is, a quantum coherence state (a dark state) and it is known that an Electromagnetically Induced Transparency (EIT) phenomenon (also referred to as CPT (Coherent Population Trapping)), in which excitation to the excited level is stopped, occurs. The frequency difference of a resonant light beam pair (the resonant light beam 1 and the resonant light beam 2) producing the EIT phenomenon accurately matches with a frequency corresponding to the energy difference ΔE12 between two ground levels of the alkali metal atom. For example, in a cesium atom, when the frequency corresponding to the energy difference between two ground levels is 9.192631770 GHz so that when the two kinds of laser light beams of the D1 ray or the D2 ray having a frequency difference of 9.192631770 GHz are simultaneously irradiated to the cesium atom, the EIT phenomenon occurs.
Accordingly, as shown in FIG. 20, when the light beam having the frequency f1 and the light beam having the frequency f2 are simultaneously irradiated to the gaseous alkali metal atom, the two kinds of light beams become the resonant light beam pair and then the intensity of light beam which passes through the alkali metal atom abruptly changes according to whether or not the alkali metal atom produces the EIT phenomenon. The signal representing the transmitted intensity of light beam, which is abruptly changed, is referred to as an EIT signal and when the frequency difference f1−f2 of the resonant light beam pair accurately matches with the frequency f12 corresponding to ΔE12, the level of the EIT signal indicates a peak value. Then, the peak top of the EIT signal is detected and the frequency difference f1−f2 of the two kinds of light beams irradiating the alkali metal atom is controlled to accurately match with the frequency f12 corresponding to ΔE12 so that an oscillator having high accuracy can be realized.
FIG. 21 is a schematic view of a general configuration of an atomic oscillator according to an EIT system of the related art. As shown in FIG. 21, the atomic oscillator according to the EIT system of the related art produces light beam having the frequency f0+fm and light beam having the frequency f0−fm with modulation applied to a semiconductor laser by superimposing the modulation signal having the frequency fm to a drive current for setting the frequency f0 (=v/λ0:v is the speed of the light beam and λ0 is the wavelength of the light beam) produced by a current drive circuit. The two kinds of light beams are simultaneously irradiated to a gas cell and the intensity of light beam which passes through the gas cell is detected by a light detector. The gas cell is configured of the gaseous alkali metal atoms and a container enclosed with the gaseous alkali metal atoms and when the two kinds of light beams which are simultaneously irradiated become the resonant light beam pair, the alkali metal atom produces the EIT phenomenon and the intensity of light beam which passes through the gas cell is large. Then, the atomic oscillator performs the detection using a low frequency signal of approximately several tens of Hz to several hundreds of Hz produced by a low frequency oscillator and controls the oscillation frequency of a Voltage Controlled Crystal Oscillator (VCXO), and outputs a modulation signal having the frequency fm via a PLL (Phase Locked Loop) so that the intensity of light beam which is detected by the light detector becomes the maximum. According to the configuration described above, the control is performed so that the frequency difference 2fm between the light beam having the frequency f0+fm emitted by the semiconductor laser and the light beam having the frequency f0−fm matches with the frequency corresponding to ΔE12, that is, the frequency fm of the modulation signal matches with the frequency of ½ of the frequency corresponding to ΔE12. Accordingly, the oscillating operation of the voltage controlled crystal oscillator (VCXO) can be very stably continued and an oscillation signal having a very high frequency stability can be produced.
U.S. Pat. No. 6,320,472 is an example of the related art.
However, in the atomic oscillator of the related art, the premise is that the EIT signal is left-right symmetrical in order to perform the control so that the peak top of the EIT signal is detected by the detection using the low frequency signal and the frequency fm of the modulation signal accurately matches with the frequency of ½ of the frequency corresponding to ΔE12. Conversely, in the atomic oscillator of the related art, when the EIT signal is left-right asymmetrical, it is possible that the frequency fm of the modulation signal and the frequency of ½ of the frequency corresponding to ΔE12 may be stable in a slightly shifted state. Even in this state, high stability of the frequency is ensured and accuracy of the frequency can also be ensured, for example, by adding a circuit which converts the oscillation frequency of the voltage controlled crystal oscillator (VCXO) into a desired frequency, to the atomic oscillator.
However, when the peak value (the intensity) of the EIT signal changes and then the degree of asymmetry changes due to an abrupt change in the temperature of the ambient environment or the like, the difference between the frequency fm of the modulation signal and the frequency of ½ of the frequency corresponding to ΔE12 changes, which causes the frequency stability of the atomic oscillator to be decreased.